In many cases where anions are extracted using strong-base resins, difficulty is experienced in eluting the extracted species. A thiocyanate strip solution has been found to be particularly useful for eluting certain extracted species but, unfortunately, the affinity of the strong-base resins for the thiocyanate anion leads to great difficulty in displacing the thiocyanate anion in order to regenerate the strong-base resin.
This leads to two major disadvantages, namely; that the capacity of the anion exchange resin (which is recycled in at least the vast majority of industrial processes) is significantly reduced (as will be seen from the later description herein) and, secondly, thiocyanate is lost and such thiocyanate is a costly reagent.
British Pat. No. 901573 to Davison and entitled "Improvements in or relating to the elution of gold bearing ion-exchange resins" thus proposed a process involving the use of thiocyanate strip solution for recovering gold from ion-exchange resins. The described method involves the use of a mineral acid (preferably sulphuric acid) to strip the thiocyanate from the eluted resin and fresh resin is then used to recover the thiocyanate from the acid liquor.
Applicant has tested this process and found that the efficiency of recovery of the thiocyanate was generally less than about 50% (and that after treatment of the eluted resin with ten times its volume of acid solution), and overall recovery of the thiocyanate from the acid liquor was ultimately less than 40%. Also it was discovered that the ion exchange capacity of the anion-exchange resin was not restored to a satisfactory extent. More satisfactory regeneration was achieved using nitric and hydrochloric acids but, unfortunately, the thiocyanate decomposes fairly rapidly in these solutions to form elemental sulphur. PG,5
The object of this invention is, accordingly, to provide a method of regenerating strong-base resins after they have been eluted with thiocyanate strip solutions wherein a satisfactory degree of regeneration is achieved and the thiocyanate is recovered for re-use, as is the resin.